第一章计算机网络基本原理与机制-教学课件.ppt

《第一章计算机网络基本原理与机制-教学课件.ppt》由会员分享，可在线阅读，更多相关《第一章计算机网络基本原理与机制-教学课件.ppt（143页珍藏版）》请在淘文阁 - 分享文档赚钱的网站上搜索。

1、1第一章计算机网络基本原理与机制2提纲网络基本概念Internet 简介协议分层和OSI 参考模型End-to-End ArgumentsInternet 设计原则3提纲网络基本概念Internet 简介协议分层和OSI 参考模型End-to-End ArgumentsInternet 设计原则4Worlds smallest web server“Cool”Internet appliancesIP picture frame Web-enabled toaster+weather forecaster6Whats a protocol?a human protocol and a comp

2、uter network protocol:HiHiGot thetime?2:00TCP connectionreq.TCP connectionreply.Get time7Whats a protocol?human protocols:“whats the time?”“I have a question”Introductionsnetwork protocols:machines ratherthan humansall communicationactivity in Internetgoverned byprotocols specific msgs sent specific

3、 actions taken when msgsreceived,or other events8Whats a protocol?协议的组成：语法（syntax)：以二进制形式表示的命令和相应的结构 语义（semantics)：命令请求、完成的动作和回送的响应的具体含义 定时关系（timing)：有关事件顺序的说明protocols define format,order ofmsgs sent and received amongnetwork entities,and actions takenon msg transmission,receipt10The network edgeen

4、d systems(hosts)run application programs e.g.,WWW,email at“edge of network”client/server model client host requests,receives service fromserver e.g.,(browser)/server;emailclient/serverpeer-peer model host interactionsymmetric e.g.:BT,Maze,e-Donkey11The network coremesh of interconnectedroutersthe fu

5、ndamentalquestion:how is datatransferred through net?circuit switching:dedicated circuit percall:telephone net packet switching:data sent thru net indiscrete“chunks”13Broadcast vs.SwitchedCommunication NetworksBroadcast communication networks Information transmitted by one node isreceived by every o

6、ther node in thenetwork E.g.,Satellite Network,CCTV-Net Problem:coordinate the access of allnodes to the shared communicationmedium(Multiple Access Problem)Switched communication networks Information is transmitted to a sub-set ofdesignated nodes E.g.,WANs(Telephony Network,Internet)Problem:how to f

7、orward information tointended node(s)Done by special nodes(e.g.,routers,switches)running routing protocols15Circuit SwitchingThree phases1.circuit establishment2.data transfer3.circuit terminationIf circuit not available:“Busy signal”Examples Telephone networks ISDN(Integrated Services DigitalNetwor

8、ks)16Timing in Circuit SwitchingCircuitEstablishmentHost 1 Host 2Node 1 Node 2between Host 1and Node 1propagation delaybetween Host 2and Node 1DataTransmissionDATACircuitTerminationprocessing delay at Node 1propagation delay18Circuit Switching:Multiplexing/DemultiplexingTime divided in frames and fr

9、ames divided inslotsRelative slot position inside a frame determineswhich conversation the data belongs toNeeds synchronization between sender andreceiverIn case of non-permanent conversations Needs to dynamic bind a slot to aconservation19A Taxonomy ofCommunication NetworksCommunication networks ca

10、n be classifiedbased on the way in which the nodesexchange information:CommunicationNetworkBroadcastCommunicationNetworkSwitchedCommunicationNetworkCircuit-SwitchedCommunicationNetworkPacket-SwitchedCommunicationNetworkDatagramNetworkVirtual Circuit Network20Packet SwitchingData are sent as formatte

11、d bit-sequences,so-called packetsPackets have the following structure:Header and Trailer carry controlinformation(e.g.,destination address,checksum)Each packet is passed through the networkfrom node to node along some path(Routing)At each node the entire packet is received,stored briefly,and then fo

12、rwarded to the nextnode(Store-and-Forward Networks)Header Data Trailer21Packet SwitchingA node in a packet switchingnetworkincoming links outgoing linksNodeMemory22Packet Switching:Multiplexing/DemultiplexingData can be transmitted at anygiven timeHow to tell them apart?Use meta-data(header)todescri

13、be data24Datagram PacketSwitchingEach packet is independentlyswitched Each packet header containsdestination addressNo resources are pre-allocated(reserved)in advanceExample:IP networks25Host AHost BHost EHost DDatagram PacketSwitchingHost CNode 1Node 3Node 4Node 6Node 2Node 5Node 726How do loss and

14、 delay occur?router buffers is fullpacket arrival rate to link exceeds outputlink capacityOutput link(loss)ABdropped(loss)if no free buffers28Four sources of packet delay3.Transmission delay:R=link bandwidth(bps)L=packet length(bits)time to send bits intolink=L/RABtransmissionnodalprocessingpropagat

15、ionqueueing4.Propagation delay:d=length of physical links=propagation speed inmedium(2x108 m/sec)propagation delay=d/sNote:s and R are verydifferent quantities!29BroadcastCommunicationNetworkA Taxonomy ofCommunication NetworksCommunication networks can be classifiedbased on the way in which the node

16、sexchange information:CommunicationNetworkSwitchedCommunicationNetworkCircuit-SwitchedCommunicationNetworkPacket-SwitchedCommunicationNetworkDatagramNetworkVirtual Circuit Network31Virtual-Circuit PacketSwitchingCommunication with virtualcircuits takes place in threephases1.VC establishment2.data tr

17、ansfer3.VC disconnectNote:packet headers dont needto contain the full destinationaddress of the packet32Packet 1Packet 1Packet 2Packet 3Timing of Virtual-CircuitPacket SwitchingPacket 1Packet 2Packet 3Host 1 Host 2Node 1 Node 2propagation delaybetween Host 1and Node 1VCestablishmentPacket 2Packet 3V

18、CterminationDatatransfer33Host AHost BHost ENode 1Node 3Node 4Node 2Node 5Node 6Node 7Host DVirtual-Circuit PacketSwitchingHost C34Packet-Switching vs.Circuit-SwitchingMost important advantage ofpacket-switching over circuitswitching:ability to exploitstatistical multiplexing Efficient bandwidth usa

19、ge ratio between peek and average rate is3:1 for audio,and 15:1 for data traffic35Packet-Switching vs.Circuit-SwitchingHowever,packet-switching needsto deal with congestion More complex routers Harder to provide good networkservices(e.g.,delay and bandwidthguarantees)In practice they are combined IP over SONET,IP over Frame Relay36提纲网络基本概念Internet 简介协议分层和OSI 参考模型End-to-End ArgumentsInternet 设计原则基本机制回顾